カブトムシ雌の闘争行動と同性間マウンティング

Abstract

Females of the Japanese horned beetle, Trypoxylus dichotomus septentrionalis, have only small horns, but they often fight over access to feeding sites by head-butting each other (Iguchi, 2007, also see Fig. 1 and the video). However, they also show intrasexual mounting (so-called homosexual behavior or same-sex behavior). In this case, the mounting female extends her ovipositor towards the mounted female's genital organ as if she copulated (Fig. 2).

Female-female mounting in this species was first reported by Iguchi (1996), and then by Tanaka (1999) and Abe et al. (2006).

My recent paper examined the relationship between the body size of females and the outcome of their fights and mountings (Iguchi, 2010). The results showed that female body size had different effects on the outcome of their fights and mountings. It seemed natural that large females more often won fights to gain access to food than small females (Fig. 3). However, it was surprising that small females more often mounted large females (Fig. 4).

Small females mounted large females significantly more often after losing than after winning fights (Fig. 5). It was interesting that small females that mounted large females after losing a fight were as successful at gaining access to food as those that won fights (food acquisition after mounting, 4/19 = 21%; after winning, 10/38 = 26%; Fisher's exact test, p = 0.75).

These results suggest that female mounting may function as a tactic for reducing intrasexual fighting between small and large females.

Fig. 2. Intrasexual mounting by females of Trypoxylus dichotomus septentrionalis. This is also called homosexual behavior or same-sex behavior. The mounting female is extending her ovipositor towards the mounted female's genital organ as if she were copulating (after Iguchi, 2010).